My parents live in the New Forest, and adders appear as soon as it starts to get warm, not just late summer. Some years ago their elderly dog got bitten by one in their garden (they didn't see it happen, but dog started to collapse and they guessed what had happened). She had to spend several days at the vets on a drip and was very poorly but did make a good recovery, so please be vigilant and if you suspect your dog has been bitten, get them to a vet as soon as possible. Adders love to bask in the sun, so are not necessarily hidden and if your dog is curious and goes to sniff or poke them it is very easy for the dog to get bitten, particularly if the adder was asleep and is therefore startled by the dog.

just to forewarn any dog owners....i saw my first adder today sunning itself on a footpath. apparently there were 3 lolling around. just in case you vary your walks in the summer to try and exclude places where they live. it seemed a bit early to me. i live in south england.